11. Spec Flashcards

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1
Q

bend vs stretch

A

going inwards vs stretching outwards

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2
Q

wavelength for alc vs CAs vs carbonyl vs N-H bonds vs sp3 C-H bonds

A

broad peak at 3300 cm-1 vs broad peak at 3000 cm-1 vs 1700 cm-1 vs same as alc (3300 cm-1) but sharp instead of broad vs 3000 cm-1

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3
Q

Range of wavelengths for fingerprint region. What’s the fingerprint region?

A

400-1500 cm-1. Region where specific vibrations = unique to specific molec –> determines identity of complex molec

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4
Q

What does IR spec measure?

A

molec vibrations such as bending and stretching, if there are vibrations —> dipole moments; most useful to detect diff functional groups, double bonds and triple bonds

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5
Q

How is IR spectra plotted?

A

% transmittance vs wavenumber; T = amount of light passing thru molec

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6
Q

How are absorbance (A) and transmittance (T) related?

A

low transmittance => high absorption –> max absorption = at bottom of valleys

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7
Q

What’s UV spec?

A

UV light passing thru a sample that’s been dissolved in inert, nonabsorbing solvent and measuring absorbance; when light passes thru sample in UV spec, e- = excited –> transition to higher energy lvl and then back down to orig lvl –> causes variable absorptions –> variable extinction coeff

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8
Q

UV spec = most useful for identifying what type of cmpds?

A

double bonds or heteroatoms (non-C or H atoms) w/ lone pairs => conjugated systems

Conjugated systems can also be: Alternating single bonds and multiple bonds; this allows pi e- to delocalized throughout unhybridized p orbitals

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9
Q

HOMO vs LUMO

A

highest occupied molecular orbital vs lowest
unoccupied molecular orbital. The smaller the difference in energy between the two, the longer the wavelengths that can be absorbed by the molecule

When e- get excited and absorbs a photon, it moves from HOMO to LUMO

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10
Q

What does position of peak (upfield and downfield) mean in H+ NMR?

A

de/shielding effects and reflects chemical environment of p+ (DESHIELD IS DOWNFIELD but deshielding = further down ppm)

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11
Q

What does area of peak mean in H+ NMR?

A

ratio of H+

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12
Q

What do # of peaks mean? Coupling constant?

A

neighboring H’s (N+1 rule: # of peaks = neighboring H + 1); magnitude of splitting is called the coupling constant, J (units in Hz)

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13
Q

How to find Hydrogen Deficiency Index (HDI)?

A
  1. find max # of H can be paired w/ C’s and subtract that # with actual # of H in formula
  2. Put the missing # of H into a pair value
  3. You can use the formula: 1/2(2C + 2 + N - H - X)
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14
Q

How to know if peaks represent CH2 or CH3?

A

Divide integration value by the lowest, the quotient must be a whole number (multiply by a factor if necessary)

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15
Q

Memorize shifts for H NMR

A

CH3: 0-3ppm, alkynes: 2-3, alkenes: 4.6-6; aromatics: 6-8.5, ald: 9-10, CA: 10-12, -OH: 2-5

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16
Q

Which HDI values tell you what?

A
HDI = 1 --> double bond
HDI = 0 --> no double bond or ring
HDI = 4 --> benzene ring
HDI = 5 --> benzene + 2x bond
17
Q

What kinds of isotopes do you need to see a magnetic moment in NMR?

A

Even numbered isotopes with odd atomic numbers or odd numbered isotopes
No magnetic moment —> no resonance detected

18
Q

Higher order bonds absorb higher frequencies on IR spec

A

Yep

19
Q

Wave number is directly proportional to? vs Inversely proportional to?

A

Frequency vs wavelength

20
Q

What do HNMR resonances mean?

A

basically means how many H’s will show up on HNMR. if H’s = equivalent –> count as a single resonance b/c there’s free rotation and it’ll show as one on NMR; if H’s = nonequivalent (ie. they have pi character) –> count as multiple resonances b/c each H has its own peak on NMR (pg 353 on princeton)